Electrophoresis is based on the principle that charged molecules or substances will migrate when placed in an electric field. Since proteins and other biopolymers (e.g., DNA, RNA, enzymes and carbohydrates) are charged, they migrate at pH values other than their isoelectric point. The rate of migration depends, among other things, upon the charge density of the protein or biopolymer and the restrictive properties of the electrophoretic matrix. The higher the ratio of charge to mass the faster the molecule will migrate.
In theory, separation of different proteins could be readily achieved in free solution provided that the molecules differed sufficiently in their charge densities. However, in practice separations in free solution are difficult to achieve. Heat produced during electrophoresis can cause convection disturbances in the liquid medium distorting the protein bands. Recognition of the individual proteins is compromised because the bands are constantly broadened by diffusion. This continues even after electrophoresis has been stopped. Therefore, electrophoresis in free solution is rarely carried out. In practice various supporting media are currently used to minimize convection and diffusion, and to effect separation both on the basis of size and of molecular charge.
Many support media for electrophoresis are in current use. The most popular are sheets of paper or cellulose acetate, silica gels, agarose, starch, and polyacrylamide. Paper, cellulose acetate, and thinlayer silica materials are relatively inert and serve mainly for support and to minimize convection. Separation of proteins using these materials is based largely upon the charge density of the proteins at the pH selected.
On the other hand, starch, agarose and polyacrylamide gel materials not only minimize convection and diffusion but also actively participate in the separation process. These materials provide a porous medium in which the pore size can be controlled to approximate the size of the protein molecules being separated. In this way, molecular sieving occurs and provides separation on the basis of both charge density and molecular size.
The extent of molecular sieving is thought to depend on how closely the gel pore size approximates the size of the migrating particle. The pore size of agarose gels is sufficiently large that molecular sieving of most protein molecules is minimal and separation is based mainly on charge density. In contrast, polyacrylamide gels can have pores that more closely approximate the size of protein molecules and so contribute to the molecular sieving effect. Polyacrylamide has the further advantage of being a synthetic polymer which can be prepared in highly purified form.
The ability to produce a wide range of gel pore sizes and to form pore size gradients within the gel are additional advantages of polyacrylamide. Control over pore size enables mixtures to be sieved on the basis of molecular size and enables molecular weight determinations to be performed. These determinations are especially accurate if the proteins are coated with a detergent such as sodium dodecyl sulfate (SDS) which neutralizes the effects of molecular charge. This technique is referred to as SDS-PAGE electrophoresis.